You are looking in the wrong place for your problem, it is a lack of volume not pressure! However even with that tiny volume you have available an impact should perform better than that, for the first few seconds anyway. You probably have a restriction in the system and if you are using the regulator that came with that little compressor then your problem is probably there. That regulator is sized to flow the small amounts of air that compressor is designed to deliver and was never meant to flow enough air to power an impact wrench, these things are for airing up tires and powering nail guns. Still if you start at the tank and make SURE that NOWHERE along the air line from the tank to the impact wrench (including ALL fittings) there is noting that has less than a 1/4" hole size then it should work, bigger than 1/4" would be even better. If there is even one spot, and it almost certainly will be inside that regulator, that restricts the air flow to less than what 1/4" can flow at 100 PSI then the wrench will starve for air and will not work no matter how much pressure it has, high pressure without enough volume is worthless!